This invention relates to regulating the output pulse of a Q-switched laser. More specifically, the invention relates to controlling a Q-switch laser output pulse by modifying the laser's resonator gain.
A pulsed Q-switched laser resonator will produce a very short output pulse of extreme intensity. The time duration of this pulse is determined by the physical constraints of the resonator; such as, the lasing medium, mirror spacing and reflectivities, excitation level, and others. These short pulses have very high peak power levels that can be detrimental to optics and to the desired effect on optic materials. In other words, the short Q-switched LASER output pulses can permanently damage optics, fiber optics, and the material to be processed.
Methods to stretch these output pulses have been reported with limited success for some types of lasers. In particular, two such techniques used to stretch a laser's output pulse are described as follows.
One method uses a high gain vacuum photodiode tube that is connected to a Pockels cell via a specially tuned circuit. When the Pockels cell is triggered, the laser begins to build up energy in the resonator and starts to lase. The photodiode detects this lasing action and feeds back a signal to the Pockels cell to slow down the rapid rise in light intensity within the resonator. The tuned circuit is coupled between the photodiode and Pockels cell to help control the amplitude of the signal from the photodiode and phase of the signal to enable the Pockels cell to stretch the resultant pulse. This method is reported by Harigel, et al. in "Pulse Stretching in a Q-Switched Ruby Laser for Bubble Chamber Holography", Applied Optics, Nov. 15, 1986. Unfortunately, this method must be constantly tuned to continue to produce the desired output pulse since other factors, such as the LASER excitation level, are constantly changing.
The second method involves a similar arrangement, but also includes a tuned circuit that applies a preprogrammed voltage waveform to the Pockels cell. This method has an added benefit that the desired output pulse shape is not entirely dependent on the photodiode's signal and the quality of the tuned circuit. Unfortunately, this method requires the designer of the tuned circuit that generates the preprogrammed voltage waveform to have an accurate model of the laser resonator's gain versus time for the desired output pulse shape. If other output pulse shapes or amplitudes are desired, then a totally new preprogrammed voltage waveform is required. Again, degradation or change of any of the laser physical constraints will incapacitate this method from performing properly.